Receivers of signals often include carrier, timing and/or phase recovery loops. A carrier recovery loop may be used to demodulate signals from a carrier frequency to a lower frequency, such as baseband. A timing recovery loop may be used to synchronize the sampling of received signals. In many devices, a phase sensing loop is used to correct phase offsets. These receiver elements or components are often useful in recovering coherently modulated signals.
One type of coherent modulation is vestigial side band (VSB) modulation in which modulated quadrature signals corresponding to data symbols are transmitted. In theory, receivers of VSB modulated signals use a Hilbert pulse to penetrate or modulate the received signals. In typical receivers, approximations to the Hilbert pulse are implemented to simplify receiver design and reduce expense. These approximations to the Hilbert pulse are often implemented as a filter in a phase sensing and correction loop.
These existing approximations to the Hilbert pulse are often based on the assumption that only phase interference is present in a communications channel between a transmitter and a receiver. Such approaches, however, may offer degraded performance, such as an increased bit error rate, when the received signals include echo signals (for example, multi-path signals). In addition, the existing approximations to the Hilbert pulse are often based on future samples of at least a portion of the received signals. As a consequence, circuits that implement these approximations often have additional time delays, which impact recovery loop stability and increase circuit expense.
There is a need, therefore, for improved receivers for receiving modulated signals that have reduced delays and offer improved performance in the presence of echo signals.